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Old 13th May 2005, 01:09 AM   #1
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Default MathCad Help?

MathCad doesn't work on my computer and I'm at a friends house and downloaded MJK's MathCad TL worksheets, which I've never used before. I'm deisnging a TL for the Hi-Vi B3N. It is going to be internally 25"x6"x6". I haven entered the T/S parameters and the enclosure geometrey information, but I don't know what to input in the entire "Transmission Line Definition" section of mathCad. Can Anyone tell me what to put in in the section "Section Length" when it asks for Lo (sub)0 - Lo (sub)4 and if I ned to change n_open from 4 and whether or not to change TR from 1.181x10^-3m?

Thanks,

Josh
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Old 13th May 2005, 02:13 AM   #2
MJK is offline MJK  United States
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I added a MathCad worksheet tutorial to the models page on my site this past weekend. You might try reading that to get a feel for what to enter and how to review the output. If you are using one of the preformatted worksheets (TL Open End, TL Offset Driver, or ML TQWT as examples), you don't need to enter any data beyond inputs required on page 1. The worksheet TL Sections is the only worksheet that requires everything be entered by hand. TL Sections is the most general, powerful, and unfortunately most difficult of the worksheets.

Also added were two short worksheets for checking Thiele / Small parameters to see if the set you have are consistent. In one worksheet you can calculate Vas from BL and in the other BL from Vas. If the two do not agree, the T/S parameters you are trying to use are suspect.
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Old 13th May 2005, 03:13 AM   #3
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Thanks Martin,

I recieved your reply email, I understand it much better, but I am still a little Iffy.

By using the quarter wave legth theory the back of the woofer you are able to excite the standing waves to the tuning frequency of the enclosure? Problem is, I still don't understand how quarter wavelegth play into this. Your site says "All of these enclosures utilize acoustic standing waves that can be described as multiples of a quarter cycle of a sine or cosine function." So the standing waves inside the enclosure will leave the enclosure (in an open TL) perhaps maybe only reaching crest by the time it exits the enclosure, while others may be more developed and be at 3/4 of a wavelegth and so on?


Usually physics comes easy to me, but I am having trouble with this, AP physics is next year.

Thanks,

Josh
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Old 13th May 2005, 12:38 PM   #4
MJK is offline MJK  United States
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Hi Josh,

"By using the quarter wave legth theory the back of the woofer you are able to excite the standing waves to the tuning frequency of the enclosure?"

The length and shape (tapered, straight, or expanding) of the transmission line enclosure set the tuning frequency. Looking in Table 1 of my alignment table documant you can see the tuning frequencies for different transmission line geometries. At this special frequency any excitation will be significantly amplified. This is true for the first standing wave, the quarter wavelength, and the series that follow, the three quarter and five quarter and so on. If you look in my tutorial, at the plot of driver and terminus SPL for the unstuffed geometry in attachment 1, you will see peaks in the terminus response at these frequencies. Stuffing is used to tame these peaks and smooth the resposne, you can see this in the plots for the stuffed line in attachement 2.


"Problem is, I still don't understand how quarter wavelegth play into this."

This is due to the boundary conditions at each end of the transmission line. For a straght constant area transmission line with one end closed (velocity is zero and pressure is a maximum) and one end open (velocity is a maximum and pressure is zero) you excite quarter wavelengths of sine (velocity) and cosine (pressure) waves at the fundamental resonance. Look at the plot on the opening page of my site for a picture of the wave shapes. For a straight constant area transmission line, the resonant frequency has a wavelength four times the length of the transmission line. The coincidence that the length of the transmission line is one quarter of the wavelength of sound at a particular frequency is what causes the resonance and tremendous gain in the sound output fomr the open end of the TL.


"Your site says "All of these enclosures utilize acoustic standing waves that can be described as multiples of a quarter cycle of a sine or cosine function." So the standing waves inside the enclosure will leave the enclosure (in an open TL) perhaps maybe only reaching crest by the time it exits the enclosure, while others may be more developed and be at 3/4 of a wavelength and so on?"

Sound is produced by a driver's motion, the velocity as it oscillates back and forth. The same is true for the open end of a transmission line, or the port in a bass reflex. The sound form the open end of a transmission line is produced by the velocity of the air moving in and out of the opening. At the quarter wavelength resonances the air moves a lot as can be seen in the SPL plot for the terminus in the unstuffed line. There is sound coming out of the open end of a transmission line, or a bass reflex enclosure, at all frequencies but the frequencies that are produced loud enough to be used are only at the resonant frequency. Put your ear close to the port of a bass reflex speaker when it is playing music and you will hear predominantly one note bass, the balance of the sound will favor the frequency to which the box is tuned. Same thing happens with a TL.


"Usually physics comes easy to me, but I am having trouble with this, AP physics is next year."

At this point, I recommend that you look at the curves abd try and develop a physical understanding of what is shown and worry about the math later. To really do the math you need a few years af calculus, differential equations, partial differential equations, matrix algebra, and a lot of numerical methods. Physics will help with the understanding but the advanced math is the key. If you understand it in words, when the math comes the lightbulb will suddenly turn on. Be patient and have some fun, doing simple experiments with speakers to see how they perform will teach you a ton.

OK, gotta go to work to pay for the speakers.

Hope that helps,
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